How to use a video as a reference on a plane?
A different approach involves not using a plane at all but an empty as an image:



Select the image in the properties->Empty->Display tab



Click on match movie length and enable Auto Refresh
You should see the number change in the Frames box on the left.



You'll get your image (or video) in an empty. The advantage of this method is that your image will be visible even in when the 3D viewport shading is set to wireframe or bounding box. Note that empties will not be rendered on the final render...